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Why does a mid-wing aircraft have lower drag?
Why does an airplane having a mid-wing have lower drag than high- or low-wing configuration even if they have  wing of same shape and size?
This article (PDF) claims it's true.

It's because interference drag (the vertical flow field forced to share space with the horizontal flow field) is minimized when the wing meets the fuselage at a 90 degree angle or more.
With a low wing airplane with dihedral, even if the sides of the fuselage are vertical, there is still less than a 90 degree angle due to the dihedral angle.  To mitigate the effect of an acute angle at the junction, traditionally a fillet fairing is installed to provide a curving transition between vertical and horizontal, reducing the intensity of the interference closest to the fuselage and minimizing flow separation.
Ideally however, it's better to have as close to a 90 degree intersection, as possible, or better yet greater than 90 degrees, and a mid wing makes this easy to do.  And if the fuselage cross section is circular, like the Grumman Wildcat, even better because the intersection is effectively a bit more than 90 degrees because the sides are curving away from the joint.  The mid wing has a number of packaging problems however (like the Wildcat's crazy landing gear, spar going right through the key cargo space on transport aircraft, etc).
The Corsair fighter killed two birds with one stone in going with the gull wing configuration; it lowered the wing at the landing gear attachment to allow a much shorter strut, and it maintained a 90 joint at the fuselage even though it was attached near the bottom.  The Corsair's gull wing achieved the benefit of a mid wing design aerodynamically, and a low wing design in its landing gear configuration.  Look at a Corsair and there is no fairing at the wing root you normally see on low wing fighters.  No fairing was necessary.
The Thorp T-18 homebuilt addressed the problem to avoid an acute angle at the wing to fuse junction, with a simple low wing, by starting the dihedral half way out on the panel, so that the mid section of the wing was flat and came out of the slab sided fuselage at 90 degrees (most T-18s use a very small fillet fairing at the back to reduce turbulence from flow separation aft of the thickest part of the wing, although some builders go with fancier ones). Thorp got the added benefit of a panel that could be built in two 4ft panels with the dihedral joint between them, allowing the wing to be skinned from a single trailing-edge-to-trailing-edge wrap of a 4 ft sheet of aluminum for each panel. Very easy to build.

"Ideally however, it's better to have as close to a 90 degree intersection as possible". Not really. The angle by itself is not the problem. Greater angle is always better. Imagine having a very thick wing: it will have >90° on both surfaces, and it will be better. (In the extreme, it will engulf the whole fuselage). The situation will be more complicated at the trailing edge though, and this is why the trailing part of the fairing is often much bigger. In a word, it's more complicated.

Yes you made a good point. I made an edit to align better with my first para where I mentioned "90 degrees or more".  As for the detailed aerodynamics behind all that, as you detailed in your answer, my intention was to keep it simple.

It's not that simple (even if we consider only aerodynamics and ignore structural and stability issues). In many practical cases, the top wing is as good or even better than mid-wing.
The culptit is, of course, interference drag: the drag that appears when we combine two or more bodies, as compared to these bodies isolated in the same stream. Like any other drag, it depends on the conditions, apart from the bodies themselves.
In some conditions, interference drag may be even negative. For example, with zero wing incidence and zero angle of attack, many configurations will have slight negative interference drag (that is, total drag of the wing+body combination will be lower than that of the sum of the isolated wing and body). In this trivial case, this is because part of the wing is covered by the body (fuselage). (Remember that by convention, wing is continuous tip to tip, including the part within the fuselage).
But the more practically relevant and interesting cases are when we have some angle of attack and lift. In this case, two main things will happen:

The airflow above the wing will be significantly faster than the free stream (as a precursor to having lift). The airflow below the wing may be slightly faster or slower depending on the airfoil and conditions. At the same time, the airflow around the body will only be marginally faster (or not at all for very slender bodies). The 'fast' flow above the wing will 'clash' with slower flow around the fuselage, creating most of the interference problems.

If the body also has a non-zero angle of attack, it will distort the flow such that the local AoA at the wing roots will be higher (up to double at the centreline without wings):

Fig. XIX-6 from Hoerner, Fluid-Dynamic Lift
This distorts the wing lift distribution, usually (though not necessarily) negatively: there is a drop of lift above the fuselage, then a sharper rise than 'normal'.1

Now, there are two main practical consequences of these effects:

The low wing is by far the worst configuration aerodynamically. Its high-speed over-wing flow is in full interaction with the fuselage. The mid-wing and high-wing are comparable in many cases, with the ideal (for drag) wing position slightly up from centre. I'll show just one of the many comparisons from Jacobs et al, NACA-TR-540, which explored hundreds of configurations:

The one that is very bad after $C_L > \approx 1$ is the classic low-wing ('Combination 187'), with the mid-wing (186) and high-wing (185) almost indistinguishable. (The former is marginally better at low lift, the latter is better in pre-stall conditions (where nobody cares about drag)).

Smooth interface (fairing) bettween the wing and the body can improve the situation dramatically, especially for the 'bad' low-wing combination. Here's another example from the same source:

It is not really the acute angle between the wing and the body by itself that creates the problem. If the wing was flat with zero AoA, this angle would not be a problem. Rather, an acute angle 'traps' the two streams with speed differences and makes them interact 'harder', so to speak. This is why high wing is more tolerant to having acute interface with smaller fairing:2 the speed difference is lower.

1 It's always helpful to remember that the section of fuselage between the wing halves does create lift - much more of it than an isolated such section, although, of course, less than a wing section. The spanwise lift distribution doesn't drop to (near) zero at the fuselage. But the distortion itself is a source of interference.
2 But in practice it often has a 'flat' right-angle interface anyway for structural reasons.

It is because of interference drag.
Wings create lift by pushing air downwards, as per Newton's third law. They do so by suction on the top, and (usually) lack thereof on the bottom. The airflow leaving the trailing edge of the wing as a downward component.

Having the wing meet the fuselage at an angle less than 90 degrees, as is the case with a wing on top or bottom of a round fuselage, obstructs this flow.
In a high-wing configuration, the downward moving airflow leaving the wing creates pressure on the top of the fuselage, creating negative lift.
In a low-wing configuration, the suction zone at the top of the wing acts on the bottom of the fuselage, again creating negative lift.

So lift is reduced.
Moreover, pressure drag is increased as the flow is squeezed between the trailing edge and fuselage (in high wings) or leading edge and fuselage (in low wings).

What's wrong with this one?

Can you describe just little why those configurations push pull the air? Not a downvoter.

What if there is no fuselage behind the wing, like in many ultralights or tandems? No problem?

There are mainly three types of drag that affect an aircraft: Form Drag, Induced Drag, and Interference Drag.  Interference drag is the drag created by two bodies that intersect with each other creating turbulent airflow.
Wind tunnel tests show that wings mounted midway on the fuselage have less interference drag.
How Interference Drag Affects Your Plane's Performance

Sorry, but "wind tunnel tests show" is hardly an answer to "why"... The question already postulates that (which is not 100% true, actually).


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Is there any specific location on airframe for gaining higher drag?
I have a model rocket and I want my rocket to reach specific height above the ground level. As soon as desired height is reached, an air brake (umbrella type) is allowed to deploy. In order to have higher drag, what is the best position for the air brake? Will I obtain same drag if the air brake is below or above the Center of Gravity (CG)? Is Cd different for different locations of the air brake (having same surface area)?
Note: Although this question is about a rocket, this really has to do with aerodynamics.

Does the rocket need to remain in stable flight once the airbrake deploys? Because deploying this far enough above the CG can make it tumble.

@AEhere supports Monica Yeah rocket needs to be stable for parachute deployment.

So here's a thought...if it's just a sounding rocket, why not just decrease the amount of fuel such that when it reaches your desired height the upward speed is minimal? I would like to understand why air brakes would be needed to arrest ascent

@JZYL for apogee precision

While this is about aerodynamics, that doesn't default it to the aviation stack. As you've noted, this is about a rocket, and I would think that [space.se] would be a good place to have this moved.

I’m voting to close this question because it belongs on [space.se].

I would keep this open, as it involves aerodynamics.

this question is clearly on the edge between both space.SE and aviation.SE. OK sounding rockets are on space.SE, but rocket aerodynamics involved on missiles are on aviation.SE. Thus, this question also belong to aviation.SE.

@FreeMan this question has no place on Space SE. DIY rocketry is fiercely off-topic there.

If you want to arrest ascent you put the airbrake behind/below the CG.  Keep in mind the rocket will tend to remain flying "nose first", which may affect parachute deployment (from nose) on the way down.
Notice the Space X Falcon 9 has theirs near the nose allowing the rocket to descend tail first, and are only deployed once the rocket has coasted to maximum altitude, slowed down, and has begun to fall. Nearer to the ground, you use retrorockets and/or parachute.
For a high altitude sounding rocket, it may be preferable to delay parachute deployment, lest the wind carry it too far away from the launch point.

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Is "wingman" strictly a military term?
Does it have to be a potential combat situation for a pilot flying on another's wing to be called a wingman. If two aircraft are being flown in formation to a destination simply for the purpose of transportation, would the term wingman be used?  Is wingman used in civilian aviation?

I've never heard the term in civilian use. It's primarily a military term, as the wingman is there to protect and to assist - it's not just a descriptor of flying in formation.

@Dan Thanks.  Is there a term a pilot would use for someone who is flying in formation with them in a non-combat situation?

Yes, someone flying in formation with a lead aircraft is called the wingman.

While the combat military role of a wingman is established, the term is entirely appropriate and is used for non-combat formation flying as well.  And unless there is a better term, it would be logical to extend the use to civilian formations also.

While the terms originated with the military, they are used in the civilian world when flying in formation. Formation flying does require training to be performed safely. As formation flying is not mainstream, getting the necessary training can be problematic.
One source is the organizations associated with Formation and Safety Team (FAST). They are "a worldwide, educational organization dedicated to teaching safe formation flying in restored, vintage military aircraft and civilian aircraft." Their training protocols are pretty much the standard for formation flying in the civilian world.
On their website they provide training and reference material. In all formation flying, there is a "Lead" and there is one or more "Wingman" or just "Wing". So, yes, the term wingman is used outside the military.

A wingman is used in a combat situation, the function of the wingman is to protect the aircraft it is assigned to. example:
Aircraft A is tasked with a reconnaissance mission to take photographs in hostile airspace. Aircraft B is assigned to A as it's wingman. If during the mission an enemy plane is spotted, the wingman (B) will do whatever it's takes to protect the mission of A.
For formation flying, there is a leader of the formation but to my knowledge there are no special word describing the roles of each aircraft in the formation.

What you are describing is not the role of a wingman, but the role of a HVA CAP.  (High value asset combat air patrol)  The roles of flight lead and wingman are typically used in the context of a same aircraft type/same mission flight of two where both offer mutual support to the other.  The designated lead in this case is generally just the most senior, most qualified pilot of the two.  But that doesn't mean that the junior wingman won't swap and lead the formation when the designated flight lead directs.  In a flight of 4 there will be a division lead, a section lead, and 2 wingmen.

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Is NASA's X-57 using a fan, a prop, or a prop/fan and what is the difference visually/physically?
Is the NASA's X-57 or the Opener BlackFly using a fan, a prop, or a prop/fan and what is the difference visually/physically?
NASA X-57 Maxwell, courtesy of Wikipedia
Opener BlackFly v3 courtesy of the Opener web site
I understand a prop is like an "airscrew", screwing itself forward through the air and taking the airplane with it. It's exit velocity is related to the inlet velocity.
I understand a fan, is used for higher speed applications, and pushes the air back, it's exit velocity being unrelated to the aircraft's speed.
I understand a prop fan is a combination of both.
What is the difference physically between a 2 bladed prop, and a 2 bladed fan?
For NASA's X-57, and other distributed propulsion aircraft, use the prop/fan to accelerate air over the wing.
I understand Illium's jet uses a ducted fan and has roughly 20 blades.
It seem's NASA's X-57 and the blackfly, are just using a 2 bladed props, or is it a fan, or a prop/fan?  How can you tell what it is?
What is the difference physically between a 2 bladed prop, and a 2 bladed fan, and a 2 bladed prop/fan, all unducted?

These are all propellers. The visual difference is

number of blades (prop: few, fan: many)
aspect ratio of the blades (prop: slender, fan: stubby)
and, as a consequence of those two points, a much higher solidity of the fan.

The physical difference is a much lower disk loading and a higher efficiency for propellers as compared to fans.

I understand a fan [...] pushes the air back, it's exit velocity being unrelated to the aircraft's speed.

Almost. While the entry speed of a propeller equals flight speed, the same would be true for a fan if it weren't surrounded by a shroud. Instead, the shroud of a fan engine accelerates the inflow at low flight speed and decelerates it at high flight speed. This greatly reduces the flow speed variation at the fan face with flight speed so that the exit flow of a shrouded fan also varies much less with flight speed than the exit speed of an unshrouded propeller.

One thing that I don't see mentioned anywhere, is that, there is only a certain range of Hp a blade can take. I think even propellers are rated by Hp/blade. So if your blades can only take 100Hp, and you have a 500 HP engine, you would need 5 of those blades. Would also appreciate a comment on my question of how the blackfly flies below 28mph with 77% less wing area...

@Fred As I understand it, the thrust of the blackly is sufficient to let it hover. In the transition the propellers greatly accelerate the flow speed over the wing so less surface is required for the same lift.

Sorry, I meant power off.  Does that mean the props are still producing enough thrust even with no power just after powering down in a demonstrated power off stall below 28mph, or are the props producing enough thrust in autorotation, or other?

@Fred With true power off the props will cause drag and will not help with lift production. The reason for super-low stall speeds with power off is usually to be found in marketing, not physics.

So you don't think it could float down to the ground under 28mph in autoration? Sort of like the AN-2 with full stick back. I've taken a stab at it here: https://aviation.stackexchange.com/q/88949/45806

@Fred No. The An-2 has slats and flaps, and its performance is sometimes greatly exaggerated. The minuscule props will certainly not make much difference.

Hmm. According to my math https://aviation.stackexchange.com/q/88949/45806, the props should be able to generate enough thrust in autorotation. Did I do something wrong?


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Can a missile retarget to the heat from the firing aircraft's engines?
I have heard that modern war planes utilize missiles which are directed to the heat of the enemy aircraft's engines.
Can a missile, shot by a fighter helicopter or jet, target the heat of the engines of the aircraft which fired it?

Not really since you have a missile lock before firing.

You mean, an accidental 'loop around', or something compared to radar targeting, where the reflection of the IR signature from the engines is reflected off the target?

Are you asking if a missle-launching aircraft can accidently shoot itself?

https://en.wikipedia.org/wiki/Infrared_homing

@RonBeyer But what happens if a friendly aircraft happens to cross the missile "lock-path" when it's already in the air and chasing a target? Infrared missiles IMHO don't have IFF.

@jklingler I'm not completely sure, but you can be sure that it isn't the shooter aircraft, just another friendly, which isn't what the question is asking.

The fly time of a missile from a MANPAD is a few seconds... there is no time for that.

Not sure about a missile, but an F11F pilot managed to shoot himself down with his 20mm cannon

@RonBeyer - Not really since you have a missile lock before firing - Not always. See: LOAL (lock-on after launch)

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Are aircraft transferable between companies with different Air Operator Certificates?
Can Company A use Company B's aircraft if the two companies are not on the same Air Operator Certificate (AOC)? Company A will provide the crew. This would be a transfer of equipment use with no transfer of ownership.
The aircraft in question happens to be a medical emergency helicopter.

I don't have the answer, but aside from AOC issues, the operators insurance company may not take too kindly to some other company's crew piloting the aircraft.

In my Australian experience, each aircraft must be listed on the operations specifications, which forms part of the AOC. However for medical and fire helicopters I believe there are exemptions in order to have an expedient process.

Would you also mind clarifying what country you are talking about?

You might want to indicate which country or jurisdiction you're asking about

The answer to this will depend on what the two company's Operation Certificates allow and forbid in the Operation Specifications (Ops Specs). All Ops Spec examples provided below represent FAA examples, are examples only, and may differ from those in any specific situations.
Considerations will include:

Aircraft Authorization
Operational Control
Maintenance
Crewmember Requirements

The Ops Specs will specify in A003 which aircraft and configurations are authorized for use, for example:

A003 . Aircraft Authorization
The certificate holder is authorized to conduct operations under the provisions of Title 14 CFR Part 135 using aircraft with the approved configuration and operations described in the following table:
[table omitted]

If the available aircraft is of a type and configuration authorized for use, then it may be possible to use it. This will most likely be the case if the aircraft is of the same type and configuration, and in the same class/category of operation as those aircraft which Company A already operates.
Ops Spec A008 will probably specify that the certificate holder shall not:

Engage in any arrangement with an aircraft owner, lessor or any other person or entity [...] which allows the use of an aircraft for operation under these operations specification without a complete, effective and sustainable transfer of operation control to the certificate holder for all Part 135 operation conducted under these operation specifications.

The lease agreement must provide Company A with complete, effective and sustainable operational control of the aircraft.
Additionally, the aircraft must conform to the extensive maintenance requirements required by Part D of the Ops Specs.
The flight crewmembers must also be trained and approved for the aircraft. However, if the aircraft is of the same type and configuration as those already listed in A003, this requirement may be easily met.

To operate B a/c at A you would either do a dry lease, but the a/c then has to be named in AOC of A so the crew of A can operate the a/c. Or do a wet lease, but then crew of A needs to fly on the AOC of B.

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How do military radars detect the type of an aircraft?
How do the military air defense radars and aircraft radars detect the type of an aircraft and enemy or friendly information?

related: https://aviation.stackexchange.com/questions/1877/how-does-iff-work

If the target is just a target (no transponder ) the the answer is probably "no it doesn't". The best the radar can do is to get the shape of the target (which itself is very difficult and infeasible until very recently). Most radars in service now can at most detect the "strength" of the reflection and can not distinguish between a small object of a bad shape(e.g. an artificial reflector) or a big object with low reflection signature (B2 bomber?) or a group of small objects (a formation of jet fighters vs a airliner).

Also related: this question

What makes you think that radar can detect aircraft type?

This depends on the type of radar system and its intended application. You can find a nice bit of history on it here. The early radars could not tell the difference between aircraft types, any kind of movement was coordinated beforehand on the friendly side and anything that was not pre-arranged was considered foe. The early Doppler systems that resided on the DEW Line could be trigger by as little as a dense flock of birds. The early radar data from the DEW Line was processed on the SAGE system a very early (very primitive) system for coordinating a great deal of weather, radar, and flight information.
After WWII and through out the cold war era newer tech was built to solve the issue of coordination. The modern Identification Friend Or Foe system that is now in use provides at least some way to pick apart radar targets.
With the advent of modern computer processing and the ability to analyze radar returns beyond staring at a scope it is possible to determine (with great certainty) the make of an aircraft by its radar signature (and in some cases thermal signature). Also known as Radar Cross Section. Everything (even stealth planes) have a radar cross section and once a cross section has been identified it could be used again to check against future pings.
One could go as far as to say there is some historical precedent around this. The general idea of "if it comes from over there is one of them, if it comes from over hear its one of us" is very much how radar worked for a long time and the identification of the actual plane was less important than identifying that it was there. If an unidentified target entered a radar area and did not identify its self, over the radio, with some kind of transponder and or at a preordained time it was intercepted. Not until visual identification was made was it not longer considered a threat.

Here in the states by 2020 the civilian world will be sending speed and position data via ADS-B OUT to supplement (and potentially replace) radar returns.

You should also address the fact IFF is used in parallel with primary radar, but is limited to confirming friendlies. No IFF response means unknown and primary radar techniques must be used to identify the a/c type.  Radar signature processing is quite sophisticated and capable (and mostly classified).  RCS has little value as it is just a reference of the reflectivity of the target related to an ideal sphere.

@fooot haha it is the best! at least for a few of us nerds out there!

Classic radar works by reflection radiowaves of objects. The time it takes for the radiowave to come back to the radar, determines the distance.
Depending on how many of the transmitted radiowaves are being reflected back, you can determine the approximate size of the object.
If an object is moving, the distance will vary, and you can calculate the speed.
You could make an educated guess as to what type of aircraft is detected by radar, based on size and speed. That is how they did it in the early days of radar.
Nowadays, detecting the type of aircraft, is done with transponders. The aircraft itself will broadcast its type and position.
Traditional radar is only used as backup on most airfields if a transponder fails.
Military uses ofcourse are different. Norad will monitor the skies for instance for any objects that cannot be identified by tranponder signals. And stealth planes will not show up on radar anyway, and will ofcourse need to identify themselves using a secure transponder.

Re "only as backup", that's not true, as there are many flying things detectable by radar that don't carry transponders.  I've several times had ATC point out a radar target, which I visually identified as a hang glider, for instance.

Radar can only detect position and speed of objects. The amount of reflected energy may be used to estimate the size of the object, but  other factors also have an impact on the amplitude of the radar echo: shape, roughness, paint, apparent angle.
Size and speed give a clue to the class of aircraft. Friendly aircraft are known, or respond, or  announce themselves. Silent unknown aircraft are potentially hostile.


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Are the size and location of the registration number regulated?
On modern airliners, how large are the registration numbers?
I know they are on the tail, but are they also under a wing?
Do the regulations vary by country? If so, what is the US regulation?

Under FAA jurisdiction, these items are regulated under 14 CFR 45.
The size of the registration markings must generally be at least 12 inches high:

§45.29
(a) Except as provided in paragraph (f) of this section, each operator of an aircraft must display marks on the aircraft meeting the size requirements of this section.
(b) Height. Except as provided in paragraph (h) of this part, the nationality and registration marks must be of equal height and on—
(1) Fixed-wing aircraft, must be at least 12 inches high

The markings must be displayed on either the vertical tail surfaces or the sides of the fuselage.

§45.25
(a) The operator of a fixed-wing aircraft must display the required marks on either the vertical tail surfaces or the sides of the fuselage, except as provided in §45.29(f).
(b) The marks required by paragraph (a) of this section must be displayed as follows:
(1) If displayed on the vertical tail surfaces, horizontally on both surfaces, horizontally on both surfaces of a single vertical tail or on the outer surfaces of a multivertical tail.
(2) If displayed on the fuselage surfaces, horizontally on both sides of the fuselage between the trailing edge of the wing and the leading edge of the horizontal stabilizer. However, if engine pods or other appurtenances are located in this area and are an integral part of the fuselage side surfaces, the operator may place the marks on those pods or appurtenances.

There are exceptions to these requirements, but these exceptions will not generally apply to the modern airliners referenced in your question.
While the regulations do not prohibit it, I think it would be unusual for the registration to be displayed in more than one location on a given side of the aircraft—for example, on both the tail and the fuselage.
Under ICAO requirements, these items are described under Annex 7.
Annex 7, paragraph 3.3.1 includes the additional requirement for heavier-than-air aircraft that the registration markings should be displayed on the lower surface of the wing structure.

3.3.1 Wings. On heavier-than-air aircraft the marks shall
  appear once on the lower surface of the wing structure. They
  shall be located on the left half of the lower surface of the
  wing structure unless they extend across the whole of the
  lower surface of the wing structure. So far as is possible the
  marks shall be located equidistant from the leading and trailing
  edges of the wings. The tops of the letters and numbers shall
  be toward the leading edge of the wing.

The FAA has no such requirement, but other jurisdictions do, such as the UK's CAA, which is effectively identical to the ICAO guidance.
The ICAO size requirements for heavier-than-air aircraft are similar to FAA requirements, with the obvious exception of the wing markings. These requirements are found in Annex 7, paragraph 4.2:

4.2.1 Wings. The height of the marks on the wings of
  heavier-than-air aircraft shall be at least 50 centimetres.
4.2.2 Fuselage (or equivalent structure) and vertical tail
  surfaces. The height of the marks on the fuselage (or
  equivalent structure) and on the vertical tail surfaces of
  heavier-than-air aircraft shall be at least 30 centimetres.

Note that 30 centimetres is approximately equivelant to 12 inches, and 50 centimetres is approximately equivelant to 20 inches.


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What is the behavior of false glideslope signals?
I know that an ILS transmits not only the correct glideslope but also signals that can be interpreted as a false glide path. But I'm not familiar with the details of these erroneous indications. What causes "side lobe" or "mirror" false glideslopes? How many are there, where are they located, and what sensing do they cause in the aircraft? What is the typical behavior of a glideslope signal outside its usable range, or is it completely unpredictable?

The glideslope system is an analog system, and as such, it's subject to aliasing, resonances, heterodyning, and eight other technical terms I pulled out of my ass.
Maybe a diagram will help:

false glide slopes
In plain English, above the "real" glideslope, there are false glide slopes caused by your equipment locking onto the wrong phase of the signals. If you lock onto the wrong lobe, it will still take you to the runway, but at a much higher rate of descent than you planned for. Maybe steeper than you can manage safely.
Assuming the correct lobe is the bottom-most one in the system (see diagram), you'll be ok as long as you intercept it from below. If you intercept from above, you could lock onto the wrong one, and then you're gonna have a Real Bad Day.

I would +1 for a great answer but not in favor of the cuss word.  So no +1.

Upvoting for the cuss(?) word.

Side lobes generally are unwanted radiations in a given direction. Smaller the antenna, smaller are the side lobes and vise versa.

Easiest way to find out false GP is ROD (rate of descent) readings. If you're on a GP and the ROD is way higher than usual, then, you're on a false GP.

To be sure, I've used the following method in my training. You receive GS signal at an altitude; to find out whether the GP angle is more or less than 3deg, take your Altitude (in feet) and divide by your DME (in Nm) x 100. Mental math

Ex: your altimeter reads 3000' and you're 5Nm to the station, and you are on the GP, then your angle of approach is 3000'/500= 6deg, therefore you're getting false GS signal. Basically Altitude divided by Range multiplied to 100, gives you approx Glide Angle.


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What is Class F airspace, and why is it not used in the US?
I've read that ICAO defines Class F airspace but the FAA has chosen not to use the airspace class in the US. What is the ICAO definition of Class F airspace and how does it differ from other airspace classes? What countries use Class F airspace? Why does the FAA only use A-E and G?

Seems like there would be too many potential legal liabilities to class F.

ICAO Class F airspace is a bit of an odd duck (and the US FAA is apparently not the only agency that thinks so - from a quick check on Wikipedia it seems more jurisdictions ignore class F than implement it. They only mention Class F as being in use in Germany and the UK).
From a functional/regulatory standpoint Class F is a sort of hybrid between "Class E" controlled airspace and "Class G" (fully) uncontrolled airspace:
Class F airspace is designated as "uncontrolled", but you can get ATC clearances in Class F airspace just like in controlled airspace. The catch is they're "advisory only" (so you don't have to comply with them, and all the weight is on the pilot's shoulders ). Similarly ATC will provide separation services to IFR flights in Class F airspace, but they do so "where possible", which means they might tell you they can't provide that service and you're on your own with see-and-avoid.
The general use case for Class F airspace seems to be allowing IFR flights to operate in "uncontrolled airspace".

As a US pilot (and thus having no experience with Class F Airspace), it sounds like the services provided in Class F airspace are effectively the equivalent of VFR Traffic Advisories ("Flight Following"): Controllers can give you advisories and make suggestions, but all responsibility is ultimately still on the pilot's shoulders, and you can politely tell the controller to get stuffed if you don't like their suggestions (without needing to declare an emergency to vest yourself with that authority).
In the US we already have the Flight Following system and culture set up for VFR folks (and generally speaking it's available nationwide, irrespective of airspace class, as long as the controller can get you on radar and isn't overworked). We also have a pretty robust Class E airspace nationwide.
Because of this the FAA probably sees no need to designate specific "Class F" regions where IFR flights can get the equivalent of "Flight Following": If you're VFR you can usually get flight following if you want it, and if you're IFR you should have no trouble staying in controlled airspace (Class A, B, C, D, or E) for your entire route of flight.

Apparently, as of November 2014, UK has reclassified all Class F airspace, leaving Germany as the lone user. I can't find a citation for it though, only Wikipedia. Unless 4.2 of this manual counts?

@rbp Does Class F provide actual separation services? My understanding was they would give you "advisory clearances" and separation "where possible" (which I interpret to be equivalent to "You're on your own - See and Avoid buddy!") but I could be entirely wrong in my understanding :)

Class F airspace is often used in the UK as a kind of "GA airway." It designates preferred paths with a advisory ATC service that GA traffic can use.
For example, there is a class F route defined between the north west of England and the Isle of Man. Having a preferred route and an ATC service makes sense for that route as its an expanse of open water. It simplifies search and rescue operations.

The UK description seems to be very close in spirit to what ICAO intends, at least the way I understand the ICAO description and based on what you're describing here. (ATC provides "advisory services", and it's probably a good idea to take advantage of those services and the preferred route which is shown by designating the corridor as Class F through an otherwise Class G region)

As voretaq7 said, it's a bit of an odd duck, and I don't know how it works in the UK but in Germany F is only to allow IFR approaches/departures for uncontrolled airfields. So basically, as soon as an airfield in G has an IFR approach, they get a F airspace around them, which in turn is usually only active when there's an IFR approach in progress. As soon as it's completed, it reverts to G (depicted on the sectional as "F(HX)", meaning it's not always active). So in order to determine if F or G rules apply, you need to ask FIS or AFIS (or approach, but then you're probably IFR already, so you don't need to care).
VFR "flight following" is not related to the airspace and is provided by FIS in E, F and G airspace, as long as radar coverage is available pretty much. In D and C it's called CVFR and is provided by approach (or "Radar" as they like to call it). It works the same way when shooting an approach into a G airfield in the US, you're only cleared for the approach, as soon as you enter G, it's up to you to see and avoid, as far as I know.
The point of having an F airspace is that the visibility requirements are different for VFR aircraft, where you have to stay a mile away/thousand feet from clouds, whereas in G they only need to stay clear of clouds. Allowing IFR approaches in G has the potential of allowing an IFR aircraft pop out of a cloud and not have even a mile or thousand feet separation from the next VFR aircraft. Note that these extra VFR requirements are specific to Germany and deviates from the ICAO definition of airspace F.
Update
Germany is in the process of discontinuing the use of airspace F for IFR approaches into uncontrolled fields, in accordance with EU regulation 923/2012 "Standardised [sic] European Rules of the Air", or SERA. Once SERA is in effect, IFR approaches will be allowed within airspace G in Germany as well. Current airspace F will be replaced with airspace G and a so called "Radio Mandatory Zone", or RMZ.

Interesting point about the visibility requirements! As a comparative data point, for IFR approaches to uncontrolled fields in the US we generally just designate the airspace around uncontrolled fields with charted instrument approaches to be Class E with the floor either at 700 feet or the surface. For example, KTPL in Texas is Class E to the surface (and surrounded by a corridor where Class E begins at 700 feet), imposing the visibility requirement along with all the other "controlled airspace" requirements.

@voretaq7; right, but there are still a bunch of fields with IAPs in G (w/o surface E), and there's nothing really stopping you from scud-running in G with a low ceiling, only to have an IFR aircraft descend out of the clouds on top of you on final. I know, I know, that's what CTAF is for, and I personally prefer the US way, but I can understand the reasoning behind why the Germans use it the way they do.

oh I definitely understand the logic behind it (a lot can happen in those last 700 feet)

@falstro if you care for another update of your answer, Germany discontinued airspace F as of Dec 11, 2014

In Canada anyway according to the CARS (Canadian Aviation Regulations) Class F airspace is "airspace of defined dimensions within which activities must be confined because of their nature, and within which limitations may be imposed upon aircraft operations that are not a part of those activities." Basically it's airspace dedicated to a certain activity and you better make damn sure you're not going to be getting in the way of that activity before you enter.

This sounds a bit like Canada is using "Class F" to accomplish what we do with temporary flight restrictions for airshows and the like in the US. Seems to depart a bit from what ICAO's "Class F" description is though - do you happen to know where in the CARS Class F airspace is described? It sounds like it would make interesting reading for a comparative study on airspace classification and utilization.

Canadian Class F is better compared to U.S. Special Use Airspace, if you read the regs for it, it covers the Canadian equivalent of Warning, Alert, Restricted, Prohibited, Military Operation, and Controlled Firing Areas. Interesting that in the U.S. we do not depict,or inform about controlled firing areas, Spotter aircraft, and controllers, prevent non-participant aircraft from being harmed. But they are depicted on Canadian charts, and basically treated like prohibited, or restricted areas as needed.

That's not always true. Restricted zones in which live-fire military training can occur can have rules denying entry to non-participating craft when active.


text:
How does an A330 detect stall without airspeed?
On Air France 447 the crew had experienced complete failure of the pitot static system, which meant they lost their readings on their airspeed indicators, but according to the Mayday / Air Crash Investigations episode the aircraft had detected that the plane was about to enter an aerodynamic stall and the stall warning systems were activated just before the crash.
How did the A330 in question detect that the plane was on the verge of stalling without the airspeed indicators working?

There are no stick shakers on A330. In normal law it will automatically work the elevators to avoid stall, but in alternate law a stall warning is all they have.

Oops, you're right. Made the edit :d

This is sad irony, we have a potential repeat of AF 447 occurring a day after I asked this question :(

Stalls occur based on a wing's angle of attack rather than the aircraft's airspeed.  (In fact, one of the basic facts that all pilots learn in their initial training is that an airplane can stall at any airspeed).  The A330 measures angle of attack using vanes mounted on the fuselage:

However, below 60 knots, these vanes become ineffective.  During the AF447 accident, the stall warning only engaged intermittently because the computers detected that the airspeed readings from the blocked pitot tubes were invalid, and therefore silenced the stall warning system.
Furthermore, because of the lack of valid airspeed data, AF447 was no longer in normal law (and therefore had no automated angle-of-attack protection), a fact which the pilots may not have been aware of.  By the time the deicing equipment had cleared the pitot tubes, the aircraft was at such an extreme pitch that airspeed indications were still considered invalid, and the stall warning system did not activate.  The computers only activated the stall warning system when the crew pitched down, which brought valid airspeed data back into the cockpit.
An Airbus press release suggests that this may have contributed to the accident:

We also note that the aircraft, or more specifically the design of the stalling warning system, misled the pilots: each time they reacted appropriately, the alarm triggered inside the cockpit, as though they were reacting wrongly. Conversely, each time the pilots pitched up the plane, the alarm shut off, preventing a proper diagnosis of the situation.

(Emphasis mine)

Which does not explain why the alpha protection can't use the angle-of-attack vane. It uses airspeed, weight and wing loading (from accelerometer) instead.

Aircraft can stall at any airspeed, the only real indicator for a stall is an angle of attack indicator. This is because weight, icing, flaps, G-forces being pulled, and speed all effect the aircraft's stall speed.
Most large aircraft, including the A330, have angle of attack indicators that detect the high angle of attack which precedes a stall. Although the other instruments had failed, the angle of attack meter was still functioning, though not very well.

To backup flyingfisch, the angle of attack (AoA) sensor is the only sensor used to determine aural stall warning. The AoA sensors were operating correctly in the case of AF447. The failure mode of sensors due to ice crystal icing is typically limited to those probes which collect by way of geometry such as pitot and TAT probes. Static ports and alpha probes seem unaffected. The aural Stall warning system in Alternate law is purely a limit AoA, and that limit is primarily determined by Mach; it reduces significantly at high mach due to compressibility.
For AF447, the stall warnings stopped when the limit of the AoA probes were exceeded. This was presumably due to a validity checking algorithm of the system to discount extreme AoA beyond which an airliner would never conceivably achieve in its lifetime whilst airborne. Although in their case, that AoA was actually correct and the aural stall warning should never have ceased. As AoA probes become increasingly inaccurate beyond certain angles, some manufacturer s choose to temporarily derive AoA from other sources such as inertials.

I don't think an AoA probe would be inaccurate unless it's passing it's max deflection.  If there are angles that become increasingly less accurate that would seem to be a serious design flaw.

They become inaccurate due to aerodynamic influences created by the fuselage, often in the presence of even small amounts of sideslip. Flight testing is conducted with probes mounted well forward into the airflow where the influences of the airframe are negligible. The results of such testing will map the effects of aforementioned influences, however they would certainly not have mapped them on an airliner at extreme AoAs evident on Af447. You would expect a fighter jet to be pretty accurate over the range of possible AoA values due to their innate requirements.


text:
Why are modern regional jet designs shifting towards under wing engines?
Historically regional jets (eg, MD-95/B717, CRJ-700/900, ERJ-145, etc) have had their engines mounted near the rear of the fuselage.  But, recently, regional jets are increasingly using an under wing configuration (E-170/175, Sukhoi SJ100, Mitsu MRJ700 to name a few.)
A couple related questions:

Why might this configuration be advantageous for small jets as opposed to the rear mounting configuration commonly used before?
If it's advantageous, I assume technical problems are what stopped it from being implemented before.  So what has changed that allows manufacturers to use this configuration now?

Related question on engine placement. It's a bit broad but maybe can add some details to your question.

Doesn't the B737 also belong up there since you put up the CSeries (Its competitor)?

@SMSvonderTann I think I might just remove the CSeries since it's a little bigger than what I was going for...  I was thinking mostly 100 seats and below, I think I got carried away :)

@JayCarr Ah, alright

@fooot I've read that one, but it doesn't really explain why smaller planes have always used rear fuselage mounts and big plane have used underwing mounting.  Let alone why there has been such a significant shift in the last 15 years.

@JayCarr, that question doesn't, but we definitely have more than one question on aircraft layout here and I am certain at least one of them does (the related questions, unfortunately, don't work very well).

Maybe this one or this one?

@Lnafziger Second one helps, but doesn't address the shift I was pointing out.

@Lnafziger Edited the title to make make it a bit more apparent what I'm after.

Falcon 2000? That's a business jet.

@egid Oops, yeah, you're right.  I got a little link happy and got ahead of myself, sorry...

Never seen an MD-95 referred to as a "regional jet" before.

@RalphJ: He probably included it due to its being the last member of the DC-9 family, which used to be (a lot!) smaller.

The underwing engine location allows for a lighter airframe.
Placing the engines under/ahead of the wings brings these benefits:

Bending relief: Engine mass is closer to where lift is created, so the structure has less stress to carry around.
Flutter suppression: Placing the engines ahead of the elastic axis of the wing shifts the flutter speed up, so the wing can be built less stiff.
Better evacuation: With wing-mounted engines a rear door is possible to create a second point for emergency egress.
Lower noise level in the rear cabin.
More useable cabin space at the rear.
Easier access to the engines for maintenance.
Shorter piping for fuel and bleed air. Especially in the narrow fuselages of regional jets the air ducts for ventilation take a lot of valuable space.
And don't discount psychology: The underwing engine design looks more like the bigger jets, which can be a big factor for some passengers. The airplane looks safer and more mature.

With modern CFD it is much easier to reduce the interference between the engine nacelles and the wing, so the biggest disadvantage of an underwing engine location can be reduced.
Note that early designs for the Boeing 737 used rear-mounted engines. By relocating them under the wings, the structure could be made 700 kg lighter. Quote from www.b737.co.uk:

Overall, the wing-mounted layout had a weight saving of 700Kgs over
  the equivalent “T-tail” design and had performance advantages.

The rear-mounted engines were a fad from the late Fifties (started by the Sud Aviation Caravelle) to the mid-sixties and driven by the concern about the higher risk of low-mounted engines ingesting debris and the higher asymmetry in engine-out conditions. With more operational experience and more reliable engines, these concerns proved unfounded. Citing www.b737.co.uk again:

Initial worries about the low mounted engines ingesting debris proved
  unfounded, this was demonstrated by the Boeing 720B whose inboard
  engines are lower than the 737's and had been in service for four
  years without significant problems.

Small business jets still prefer rear-mounted engines so they can operate from more airfields (on which the runway might not be as clean as those of big airports) and because the weight penalty of rear-mounted engines is less pronounced. Instead of bending and thrust loads, handling loads and manufacturability drive the minimum skin gauges on smaller jets.

Does the smaller size play a part in putting engines on the rear? Putting high bypass tf engines under a low wing on a small aircraft seems like it might require an unusually tall undercarriage

Seems also like it's possible that the early generation of RJs were all adaptations of business jets (CRJ derived from Challenger) or turboprops (ERJ-135/145 derived from EMB-120, directly targeted at CRJ) while the new generation are all clean-sheet designs. By starting fresh, they chose the more optimized layout, perhaps based on experience with the older designs.

@egid - You might consider developing that into an answer.

Peter, this explains why one might use that design, but it doesn't explain why it wasn't used before, but now is being used widely for 50~100 passenger jets.

@JayCarr: Note the sentence about modern CFD: Older designs would damage the flow around the wing much more by placing the engines on the wings. Read this about the cut-back pylons of the DC-8. The designers of the Caravelle, the Tu-134, the Trident, the 727 and what not wanted to avoid the troubles Douglas had to go through.

There are a few reasons for the placement of the engines under wings. As most of the pros and cons have been already discussed elsewhere, I don't want to go into them, but I'll just point out some reasons for this trend.

One important shift in the last two decades in airline industry is the use of high bypass engines, which have higher diameters and are quite difficult to place in the rear. As the trend appears to be towards higher bypass ratios, it makes more sense to put the engines under wings where engine size increase could be better accomodated.
Another point is that the aircraft today are not developed seperately for regional and national use, but developed as part of a family, which can be used in a variety of roles. Flighglobal points this out on its article on the Embraer E170:

The response reflects the shift in emphasis since 1998 away from a product geared solely to the regional airline market to a more robust family design with equal appeal to traditional mainline carriers.

Note that the 170 is not called ERJ 170, just the E-170; the dropping of the regional moniker is significant. Embraer considerd using a tail mounted engine for 170, but went with a wing mounted one as this design proved better.

The mounting of engines on the wing helps in reducing weight. Mounting engines on the tail needs beefier fuselage for thrust transfer and usually necessiates a T-tail due to engine location. This is somewhat cancelled by a shorter landing gear, but as the aircraft size increases, the weight savings become significant.
From the airline/customer point of view, the mounting of engine on the wing is better in two respects- reduced maintenance due to engine accesibility and reduced noise in cabin.

Could you possible expand a bit on what you mean by the engines being too large to mount on the rear?  Is there some magical diameter where the lever action on the mounting braces would be too much to safely keep the engine attached to the fuselage?  And if so, what is that diameter and why does it exist?

To answer your WHY, these are larger regional jets. Most regional jets are too small for engines under the wing.
As theses jets get larger (mini-737) there is such room. Thus these designs can take advantage of the benefits of wing mounted engines.
It's unlikely you will see many jet aircraft of the size of the Embraer ERJ series (30-40 passengers) with under wing engines.


text:
Why are the pitot tubes on supersonic combat aircraft so long?
I've noticed that on combat aircraft the forward (nose-mounted) pitot tube is usually very long, on the order of half a meter to well over a meter on some aircraft. Why are they so long? Surely the body of the aircraft is not disturbing the air in front of the craft, neither during subsonic nor supersonic flight.
Example, from an AIDC F-CK-1:

Example, from a Mirage III:

Note that I couldn't find any good pictures of the Mirage in which the entire pitot tube actually fit in the photo!

TIL: Close up photos of F-16s make for quick instant StackOverflow karma.

Isn't the one on the front of the Mirage III a refueling probe?

@JonStory: No.

My bad :p I could have sworn I read something about nose refueling probes on a Mirage though

Some do not... USAF F-4C/D and Navy F-4 variants had pitot tube on the leading edge of the vertical stabilizer.

The length is not needed for supersonic flight, but for subsonic flight at high angles of attack. Due to the high wing sweep, a high angle of attack capability just happens to coincide with supersonic configurations.
In subsonic flow the air ahead of the aircraft is influenced by the aircraft's pressure field, and at high angles of attack and high wing loadings this reaches out quite a bit. The pitot tube can only measure total pressure when it points into the flow direction. Ahead of the aircraft, the local flow angle increases the closer you are to the aircraft, and this increases measurement errors, because now the pitot tube sits at an oblique angle to the airflow. A longer pitot tube reaches farther out into still relatively undisturbed flow, so less compensation is needed to arrive at good values for total and static pressure. In early flight test, the pitot tube is much longer again, because the compensation factors are not yet established.

Thank you Peter! I had noticed as well that early development aircraft have longer pitot tubes. Thank you for taking the time to explain the issue and include so much detail.

Actually, the aircraft is distorting the airflow in front of itself. As speed goes up, the shockwave builds and the aircraft starts to experience ever more sonic drag.
The long pitots will pierce through this thick boundary layer, into the smoother airflow in front, giving correct readings.
Physics Central has some Schlieren photos of the effect. When breaching the "sound barrier" the shockwave separates, which causes the sonic boom.But an area of distorted air remains.
The image below shows the effect on a space launcher, the escape tower sticking out through the separating shockwave.

Thank you, I did not realize that the disturbance could stick so far out forward. That is the Ares rocket, no?

@dotancohen I don't know, just found the image looking for something that'd illustrate the idea

@dotancohen A quick reverse image search says that it is indeed the Ares I-X rocket.

@jwenting, your explanation seems fairly sound, but I know for a fact that calculating airspeed with a pitot tube from supersonic flow is a completely different thing than for sub sonic flow, see these lecture notes. So althouh the pitot tube may be protruding through the bulk of the shock wave, there is still a (smaller) shock wave in front of the pito itself.

@Michael_K of course, but that'd leave a smaller inaccuracy due to things like turbulence and boundary layer separation than closer to the main shockfront.

I agree having the pitot out of the bulk of the shock would be very beneficial.

The picture shows a condensation collar, an effect typical for high subsonic speed. And on pointy aircraft noses airflow does not detach; detached shocks are typical for blunt configurations like the space shuttle.


text:
What is the function of variable inlet guide vanes in terms of pressure change?
Nowadays in the turbofan Variable Inlet Guide Vanes are more and more used. My question is: which is their function in terms of pressure change?
The question is very specific: I know that they allow to give the most proper direction to the fluid, but do they act also to increase the pressure? So: what happen between the inlet and the outlet of each blade, talking in term of pressure?
I need some reference, if possible.

Possible duplicate of What is CVG in turbofan engines?

Voting to leave open as not a duplicate since this question is asking about the specifics of pressure change effected by the VIGV system.

You need to be careful in understanding the significance of the variable guide vanes (VGVs) being variable.  You could think of them as a bit like venetian blinds -  when they they are most open they present the least obstruction to the air flow, much as you get the most light through a set of blinds when they are at the right angle to the incoming light. As you close them, you get less flow (light) through them, as they present a greater obstruction to the flow.
The VGVs are only variable to maintain stable flow in the compressor at conditions away from the design condition.  The design condition for jet engines is always high power during cruise (or at the top of the initial climb to be pedantic), so the VGVs only start to close at reduced powers.
At high power the vanes are at their most open to maximise flow through the engine, minimising pressure losses across them and maximising thrust.   As they are gradually closed when power is reduced, they present more of an obstruction to the flow (acting as effectively a variable area throttle valve in the path of the flow being closed down), so presenting an increased pressure ratio loss (or if you prefer reduced pressure recovery).
The increased use of VGVs really shows designers pushing at the limits of airflow stability in pursuit of better efficiency by maximising pressure ratio per stage at the design condition, at the cost of the increased weight of the mechanisms to drive the VGVs.
Not sure I expressed that very well, but first post on stack exchange.

Some references at https://www.researchgate.net/post/What_is_the_key_importance_of_Variable_stator_vanes_in_an_axial_compressor_of_a_gas_turbine   Kerrebrock, J .: Aircraft Engines and Gas Turbines, 2nd Ed.
Dixon, S.L. .; Hall, C.:Fluid Mechanics andThermodynamics of Turbomachinery, 7th Ed.

Variable Nozzle Guide Vanes do the same job as non-variable ones, they are just optimised for a greater range of conditions so their characteristics will be the same.
From this article:

They direct the airflow onto the turbine blades while at the same time converting pressure energy into kinetic energy.

They will accelerate the flow and therefore reduce the pressure at the vane outlet.
There is also this blog, which is a very good and informative read about gas flow in a turbine.

Hey Peter, that refers to nozzle vanes

VIGVs are similiar to IGVs with one major exception:
VIGV's move either mechanically or hydro-mechanically to increase or decrease angle of small blades (located at front of compressor) to maximize airflow into the compressor. Doing this allows airflow to not shock the 1st stage compressor blades which can minimize things such as compressor stall due to speed of aircraft. The faster the air enters the compressor the more chance there will be to stall the compressor/engine.
Google Pegasus turbo fan engine (F402-RR-408) to see how the VIGV's help redirect the air on the dual shaft, counter-rotating compressor/fan. This is an amazing engine that counter rotates to help with torque while the Harrier is in a picture perfect hover.
If the motor did not have this feature the entire fuselage would want to rotate in the direction of the spinning blades inside of the motor. So the first section of the motor is the first of 3 stages of stators followed by a rotating stage of blades alternating by stator/fan/stator/fan blade which are clockwise spinning blades.
VIGV's redirect airflow counter-clockwise to enter the first of 8 stages of clockwise rotating blades.
To my knowledge, this is the only counter rotating motor.

There are some recently developed turbofan engines which use contra-rotating spools: https://aviation.stackexchange.com/questions/39285/why-does-the-lp-compressor-rotate-in-the-opposite-direction-of-the-hp-compressor/39307#39307


text:
Is the "glory" optical phenomenon frequently seen by aircraft pilots?
A glory being this optical phenomenon:

(Source)

It reveals the presence of supercooled water, a hazard for an aircraft (see answers).
Due to the way it can materialize, the center of the rings indicates the position of the observer in the airliner. So this one was not shot from the cockpit.

Does it represent a specific icing hazard?
How frequent is it?

Short Answer:
Yes, pilots frequently see this phenomenon.
No, it does not necessarily present a specific icing hazard.
Yes, in some circumstances it can be in indication of an icing hazard.
Longer Answer:
Speaking for myself, yes, I often see this. I cannot speak for all pilots, but I think many pilots do also see this. The phenomenon depends on several conditions, the primary ones being angle of the aircraft relative to the sun, and the presence of liquid water droplets.
However, I would contest the assertion that it reveals the presence of supercooled water. I see the phenomenon most often in low level fog banks or mist. I do a lot of early morning flying when the conditions are right for the phenomenon. On many of these occasions the air temperature is well above freezing, and all other weather conditions would also indicate that the water droplets are not supercooled. See the following photos from the wikipedia page showing the phenomenon in low level (probably above 0°C) fog banks:

Since the water droplets connected with the phenomenon are not necessarily supercooled, the risk of icing will depend on other factors, primarily outside air temperature. Though the "glory" phenomenon does indicate visible liquid water droplets, it does not necessarily indicate an icing risk. A thorough pre-flight risk assessment for icing conditions should answer that question on a case by case basis.
Now, the conditions in which many airline passengers might see the phenomenon may well be at an altitude high enough that the liquid water droplets may very well be supercooled. In such cases the "glory" phenomenon indicates that the cloud in question is not formed of ice crystals (which pose little icing risk) but of liquid, and possibly supercooled liquid droplets.

Thanks for correcting a statement I may have misunderstood (#5 here)

@mins Yes, that paragraph began with the conditions of high altitude and low temperature.

Yes. It represents a potential icing hazard. As you noted, the glory (or pilot's halo or glory of the pilot) indicates the presence of liquid water in the cloud. If the temperature is in the freezing range and the aircraft descends into the cloud where glory is seen, icing can be expected.
From NavCanada manual:

The glory is one of the most common forms of halo visible in the sky. For the pilot it is a warning sign of potential icing because it is only visible when there are liquid water droplets in the cloud. If the air temperature at cloud level is below freezing, icing will occur in those clouds that produce a glory.
A glory can be seen by looking downwards and seeing it surround the shadow that your aircraft casts onto the cloud tops. They can also be seen by looking upwards towards the sun (or bright moon) through clouds made of liquid droplets. ... . Although ice crystals often produce other halos
and arcs, only water droplets form bullseyes.

Glory is quite common. In pre-airplane days, glories were quite rare and one good way to see them was to go up a mountain, a phenomenon called Brocken Spectre. However, with the advent of air travel, the phenomenon has become more common.

Your Google skills are strong, but It does not necessarily represent a hazard. It only means that liquid water droplets are present in the cloud or other form of visible moisture, as opposed to the cloud being comprised of ice-crystals.

@JonathanWalters I see your point. It would be more appropriate to call it a potential hazard. Also, the air temperature is also noted as a precndition for the icing hazard.

@aeroalias yes, but that is in no way specific to the glory.  Just about any clouds aside from cirrus and upper tropospheric stuff will contain liquid water.  You don't need a glory to tell you a cloud is made of liquid water, just looking at the cloud itself will tell you that.


text:
Can a closed Airworthines directive get reopened by a new revision?
There is an Airworthiness Directive that has been closed by accomplishing its terminating action. But there's a new revision that changes the terminating action. Do I have to comply with the new terminating action to close the AD?

When ADs are revised or new ones issued over the same technical issue as the original, there will normally be language in the compliance requirement to the effect that aircraft that were compliant with the terminating action of the previous AD may be considered compliant with the new one.
If it's not there, such as where a new risk factor was exposed that was not mitigated by the original AD's compliance, you have to treat it as an all new issue.  However, where a previous compliance provided a partial mitigation to the new situation, usually there is credit given for that.
There is a fair amount of effort made in crafting these things to minimize the operator burden and avoid unnecessary repetition of work as long as the related risks are dealt with.

ADs aren't 'closed', they are 'complied with'. If the AD has been revised and the new compliance criteria are not met, then you would need to perform whatever measures are required by the revised AD.
There are a number of reasons that the AD may have been revised but in the example that you allude to, the proposed action does not appear to be sufficient to remedy the problem. Therefore a new or revised AD has been issued.
If the revision is mostly a clarification or change in affected aircraft, the identifier usually remains the same e.g. 2018-03-03 R1
If the revision makes a substantial change to the method of compliance, they usually rescind the original AD and issue a new one. e.g.

SUMMARY: We are rescinding an airworthiness directive (AD) for Lycoming Engines model TIO- 540-A series reciprocating engines. The existing AD, AD 71-13-01, was prompted by a report of a failed fuel injector tube assembly. Since we issued AD 71-13-01, we became aware that Lycoming Engines no longer supports Service Bulletin (SB) No. 335A, which was incorporated by reference in AD 71-13-01. The intent of the requirements of that SB is now in Lycoming Engines Mandatory SB No. 342F, which we have incorporated by reference into AD 2008-14-07. The FAA determined, therefore, that this requirement is duplicated by another AD.

I had my A&P comply with a fuel selector AD 2018-02-05 so it was noted as complied with in my logbook. The AD was superseded by AD 2018-07-03 which allows a pilot to comply with the AD, so I made an entry in the logbook to indicate that I had complied with the new AD. This example is the same as the one in your question, though in my case the only additional action required was a logbook entry.

SUMMARY: We are superseding Airworthiness Directive (AD) 2018-02-05 for certain Piper Aircraft, Inc. Models PA-28-140, PA-28-150, PA-28-151, PA-28-160, PA-28-161, PA-28-180, PA- 28-181, PA-28-236, PA-28-201T, PA-28R-180, PA-28R-200, PA-28R-201, PA-28R-201T, PA- 28RT-201, and PA-28RT-201T airplanes. AD 2018-02-05 required inspecting the fuel tank selector cover to verify the left and right fuel tank selector placards are located at the proper positions and replacing those that are improperly located with new placards. This AD addresses the same unsafe condition and requires the same actions as AD 2018-02-05, but changes the inspection of the fuel tank selector cover to a preflight check and allows for various fuel selector clocking configurations. This AD was prompted by our determination to allow the owner/operator (pilot) holding at least a private pilot certificate to perform the preflight check. We are issuing this AD to allow the pilot to do a preflight check of the fuel selector placards.

FAA issuing AD revisions is a new one to me. I retired from an OEM a few years ago though and it was always a pain in the buns because the FAA wouldn't revise ADs even for minor changes; it always had to be a new issue.

@JohnK There aren’t many of them; I counted 3 for 2018 and they appear to be for minor corrections e.g. wrong part number, clarify wording, etc. Anything major like adding models or changing compliance requirements results in cancellation of the original AD and issuance of a new one. All of the revised ones have a -R at the end. You can find them by clicking on 'Current ADs->By Number on this page:  http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAD.nsf/Frameset?OpenPage


text:
What body creates legally binding aviation regulations in Europe?
Currently trying to get general picture of aviation organisations in Europe. At what stage do regulations get the status of a law?
ICAO issues SARPs, they are certainly not a law. Then, on the basis of SARPs, EASA issues legislation. Still, EASA is not a regulatory body.
Consider PART FCL, for instance. Does it have the force of law, or is it just a guidance for the local CAA to issue a regulation, which will be considered as law only for that specific country?

EASA is a regulatory body, though it does not regulate all aspects of aviation in the EU. Its predecessor, JAA, was not a regulatory body.

I think the legal bindingness of ICAO SARPs is still being a debated issue among lawyers.

EASA regulations,  as emited by the European Council, automatically become law in all EU member states (it is part of EU adheration treaty).
PART-FCL, for instance begins with this text:

COMMISSION REGULATION (EU) No 1178/2011
of 3 November 2011
laying down technical requirements and administrative procedures
  related to civil aviation aircrew pursuant to Regulation (EC) No
  216/2008 of the European Parliament and of the Council

Because of the way these regulations are framed, local regulations (emited as law by local parliaments) will take precedence over regulations emited by the European Parliament, so some local authorities (I’m looking at you UK) will amend/change/adapt these regulations as they see fit.
Make no mistake, EASA regulations are law within the European Union’s member states.

So, EASA regulations are something, which national authorities should respect, creating local rules. But they are not legally binding for anyone.
A regular person have to follow the local regulations, not EASA's one. If I will directly follow the PART FCL, I can expect to break some local laws one day. When EASA will issue a new regulation - I shouldn't follow it straight away, but must wait until this new rule will be incorporated into the national regulations. Is that correct?

no, EASA regs ARE legally binding and constitute law! Local laws can override it, and as far as I know (unlike ICAO) the local regs can be even be contradicting general EASA rules, so yes, you might break the law by following PART FCL directly, without checking local regs aswell

@Radu094, EASA regulations are non-binding (according to EASA themselves). See https://aviation.stackexchange.com/a/55896/19

Legally binding aviation regulations in Europe are created on a national level. Each country has their own regulator. So there is no pan-European legal set of regulations, although EASA regulations does provide a framework for some topics.

Can you identify some of the national regulators?

@jjack There are 51 states in Europe, I don't really think a list of all their regulators would fit this format

I just meant one or two. Where're you from? Can you give the entity in your country?

https://www.faa.gov/aircraft/air_cert/international/bilateral_agreements/link_intl_sites/civil_avi_auth/

Argentina Administración Nacional de Aviación Civil_____Australia Civil Aviation Safety Authority_______ Austria Federal Ministry for Transport, Innovation & Technology _______Belgium Direction Generale Transport Aérien _______
Brazil  Departamento de Aviação Civil _______
Canada  Transport Canada Civil Aviation Authority


text:
What is the gross weight limit for a Light Sport Aircraft?
In the US, I see a lot of LSAs stating their take-off or gross weight limit as 1320lbs.  Can some of these planes have a higher physical limit and that 1320lbs is just more a category limit?

Yes, 1320 pounds is the regulatory Gross Takeoff Weight for Light Sport aircraft.
From the AOPA website:

Light sport aircraft are defined as simple, low-performance, low-energy aircraft that are limited to-
1,320 pounds maximum takeoff weight for aircraft not intended for operation on water; or
1,430 pounds maximum takeoff weight for aircraft intended for operation on water.

1320 pounds is the weight limit for land-based LSAs, or 1430 pounds for sea planes. The regulatory definition is in 14 CFR 1.1:

Light-sport aircraft means an aircraft, other than a helicopter or
  powered-lift that, since its original certification, has continued to
  meet the following:
(1) A maximum takeoff weight of not more than—
(i) 1,320 pounds (600 kilograms) for aircraft not intended for
  operation on water; or
(ii) 1,430 pounds (650 kilograms) for an aircraft intended for
  operation on water.


text:
What is the reason behind the colors used for different flight categories?
In Skyvector, Foreflight and Aviation Weather Center the following colors are used for flight conditions:

LIFR = Magenta
IFR  = Red
MVFR = Blue
VFR = Green

In most contexts, red is used as the color for the most caution/hazard, but LIFR has lower weather conditions than IFR. Why use pink for LIFR?
Somebody may answer: because LIFR is a subcategory of IFR (or MVFR is a subcategory of VFR).  Are there any other IFR subcategories (other than LIFR) to justify branching off that category? It seems like a creating a subcategory branch should have at least two categories.


text:
Is it allowed / legal to produce a Type Rating CBT without the manufacturer's permission?
Pelesys, Navcon, Flyco, CPAT Global - just to name a few companies - all provide and sell various aircraft Computer Based Training Programs.
Is it allowed / legal to program and sell products that inform and teach aircraft systems and operating procedures without permission from the aircraft manufacturer? How do they get the approval (if there is any approval required)?

Which country or regulations are you asking about?

No specific country. O

No specific country. For example company X from any european country wants to produce and sell an Airbus CBT. Are they even allowed to create a product and give instructions about the aircraft - or do they have to get approval from the manufacterer?

Sure, they can do it, but without the approval of the manufacturer they'll have a hard time getting anyone to pay for it, and if the CBT is intended to be part of a required training program, it may not get very far without the manufacturer's blessing,.


text:
Do small civilian aircraft have autopilots?
If not, why? Doesn't it make flying safer? It seems to me that automating a small plane should be easier than automating a car (excluding dealing with Air Traffic Control). Is it too expensive?

My 1978 Cessna 177-B has an autopilot, an STEC System 50.

Note that your comparison to automating cars does not take into account that plane automation has been around for over 100 years (the first autoleveler was used in 1912). While car automation had the earliest ideas in the 1920s, most ideas until roughly the 1980s involved remote controlled cars or cars that require some sort of guidance circuit INSIDE the road, effectively requiring the replacement of millions of miles of road.

It should be mentioned that current automation in civilian aircraft are more "target value following" (e.g. following a predetermined route) than "decision making" (e.g. adjusting route for favorable winds or to avoid turbulence), so if your example for car is a driver-less car, then there is no autonomous aircraft yet, except experimental or military autonomous drones.

The question of whether or not automation makes an aircraft—much less a small, single pilot aircraft—safer is a very complex and broad question. If it is answerable at all in this context, it should probably be a separate question.

@Jonathan Walters but also an interesting one. I'll post it as a separate  question

Your comment: I was wondering if there are systems which could theoretically fly without Assistance from after takeoff to before landing, doing everything a pilot can do, like avoiding turbulance, Physical obstacles and dealing with emergencies." is very different than what your current question asks for. No current answers deals with this aspect (but they are upvoted). Please edit your question to clarify.

Your question sounds like you're referring to Tesla's autopilot, or the ability to guide itself without any pilot input through complex situations.  Planes already have features roughly analogous to Tesla's autopilot, but fully pilotless planes are still a long ways off.See this question for more details: https://aviation.stackexchange.com/questions/1802/why-do-we-still-use-pilots-to-fly-airplanes

There's some terminology in aviation to be aware of, with an "FMS" navigational computer that let's you enter navigation points and from that computes flight segment paths, a "flight director" that displays what attitude to hit to fly the desired path, an "autopilot" that auto-trims and also computes the way the plane should fly to follow either the flight director commands or the FMS commands, and a fly-by-wire system that moves airplane control surfaces to reach the desired roll rate, vertical speed, etc.  There's some overlap in the last three systems.

@Cody Thank you for the reference. That thread essentially answered my question. Out of curiosity, can you give me a comprehensive reference(s) to learn about aircraft design and operation?

Many small, light aircraft are equipped with an autopilot today. They can range from very simple autopilots, without altitude selector and so on,

Source, ST SYS 30, $ 12,000

to advanced systems with altitude selector, automatic trim and so on.

Source, Bendix/King KFC 225, $ 15,000 - 42,000

And with this GPS, feeding the autopilot with data, it has the abilities of an airliner.

Source, Garmin® GTN™ 750, $ 17,000

You have to know, the systems in the general aviation industry are getting better and better. Of course they are way below the complexity of the avionics of airliners, but the features are the same if not more.
For example: A captain of an airliner has to review or change the flight plan with the FMC, respectively MCDU on Airbus, a not very straight-forward device to control the plane.
Of course, the FMS has a lot more capabilities than a GA GPS, but one of it's primary features is the input of the flight plan. And in that, from the usability-view, the Garmin® GPS has the better solution: An all-in-one navigation device with touchscreen input.

What an incredibly lucrative software field.

I don't know that it's lucrative. I'd guess it's expensive (high cost to develop and, especially, to QA, and maybe even to sell or install) and perhaps few sales.

@Fattie If you are interested, I have already asked a question on this: https://aviation.stackexchange.com/questions/38122/why-are-general-aviation-avionics-so-expensive

great QA @NoahKrasser, thanks. I can tell for a fact that in one of our particular software specialties, the company, which, does "the same as everyone else" but operates in the "aircraft instruments" field - makes spectacularly more money than the rest!  :)

@ChrisW Not to mention high risk of liability if you get anything wrong and with software, you almost always do.

@Fattie No, "Angry birds" is an incredibly lucrative software field. If you ever get involved with developing safety-critical software, you will find out why these guys don't make much money at all. But if they cover their costs, getting the product out there is very well targeted advertising that raises brand awareness for their commercial-aviation products.

@Fattie: As with anything in aviation including software: if you want to make a little money start with a lot of money. Getting things approved and certified is expensive and can cost more than all your initial investment + developer time. And the thing is, it is legally mandatory - you cannot sell it if it's not approved. Chinese companies like Yuneec have been trying for years finally giving up and selling small drones instead (almost no manufacturer regulations, all regulations regulate owners)

hi @alephzero, you'd be surprised, behind the scenes one of the biggest earners on the "app store" is a serious aviation product.  folks are always amazed to learn that Scrabble (yes - Scrabble!) is one of the biggest earners on that venue also.

@slebetman - right, regulatory is the gateway

Yes.  Small airplanes can have autopilots.  They are very expensive (~$20,000) but are occasionally added to planes as aftermarket modifications or as updates to existing systems.
Autopilots in small planes range from simple wing levelers to fully coupled systems that can use GPS systems fly instrument procedures all the way to published minimums (200' above the ground in some cases).
I owned a small four seat plane with an autopilot that could hold altitude and a heading -or- a VOR/LOC course -or- a GPS track.  It was considered a relatively basic system.

Does it make flying safer? That is a very interesting question that is open to debate. The great thing about an autopilot is that when turned on, it frees up the part of your mind you are using to control the aircraft to actually think about other things such as where you are going. The bad thing about autopilots is that they are complex and need to be set correctly (and can fail). So you need to be more familiar (or current in pilot speak) with the make and model of autopilot, GPS, whatever, in your aircraft. Years ago when I learned to fly the CAA here in the UK didn't encourage the use of Autopilots or GPS. Should they? Like I said it cuts both ways. Before I get loads of comments saying what a dinosaur I am etc etc. I would like to refer to one example I read about in a light aircraft where the autopilot was set to altitude hold in cloud and the engine failed. The autopilot held the altitude and the aircraft stalled, spun, and crashed. For sure they have their place but I just don't get the button I can press and then I don't need to do anything mentality. Maybe thats just me.

The kind of autopilot I was thinking of would detect the failure of the engine and take immediate action. What you(and I think the other answers too) are describing is more like cruise control in cars than the full automation I was imagining. I was wondering if there are systems which could theoretically fly without Assistance from after takeoff to before landing, doing everything a pilot can do, like avoiding turbulance, Physical obstacles and dealing with emergencies. A plane equivalent of what Tesla is trying to do with their cars.

@A.W.Grossbard - how exactly would you propose the Autopilot handle engine loss in a single engine aircraft?  Are you going to map the world and pre-score it for off field landing suitability?  What if there's a ballgame in progress or a schoolbus making its way down your highest scored automatic choice?

@Chris As you can probably tell, I have no experience with aviation. But my guess is that the autopilot would take whatever actions are necessary to enter into a controlled descent, identify potential landing/ crash landing sights, select one, and direct the aircraft towards it. You would not need to "prescore" the world for this, you could use google maps to find flat and inactive areas(google actually has live use data collected using Android). And I really don't think pilots are capable of detecting whether a  ballgame is in progress and adjusting their flight path as their plane crashes.

@A.W.Grossbard the fact that you have no idea what you are talking about is overwhelmingly evident.  Try a discovery flight sometime, it's not expensive.  Should you take lessons, what to do when the engine stops will be a practical part of training from relatively early on.  Far before anyone would propose automating that, they'd probably just install the existing technology of a ballistic parachute for the entire light aircraft.

@Chris They're like $150 dollars(from cursory googling)! That's too expensive for me at the moment. If you wouldn't mind telling me, what part of what I wrote is obviously insane? There is a standardized procedure for dealing with engine failure and crash landings, right? I'm guessing your thinking that the "just use google maps" part of my comment is stupid, because it is. Disregard it. I get that not killing tons of people when you land could be an issue for the computer to deal with.Do you by chance have a general aviation reference you could refer me to to learn more?

@Chris That actually makes flying seem more safe to me. Thank you for taking the time to respond to my question and comments.  I appreciate your expertise and just want to learn more.

Yes autopilots are like cruise controls that follow preset paths. There are some exciting developments like having the screen in front of you show you the scenery outside with the runway on it, even though you are in cloud so you don't have to do all the maths of working out where you are. To me that is amazing, but as far as having autonomous aircraft without a pilot, that blows my mind. Even if it could be done in what circumstances will it sacrifice me to save more than one person on the ground? I think technology is a good as a servant, I'm not sure I'd like it in command personally...

To answer the why part of your question, turn it around.  Most of us who fly small airplanes do so at least in part because we enjoy doing the flying.  Why on Earth would we want an expensive device to keep us from doing what we are paying good money to do?  We might as well just fly commercial :-(
Indeed, I honestly don't see why Tesla is wasting all that money trying to develop self-driving cars.  It just ensures that I'll never buy one.

Well it's fortunate that you are capable of driving reliably, but there are some people who have limited vision or physical handicaps. Not to mention the many people killed by drunk drivers each year. It's definitely not a waste of money.

I will add that most people drive cars just to get somewhere. In that respect, an automobile autopilot is not only desirable to the person, but desirable to everyone else from the elimination of a distracted driver. If they get it to work... an automobile autopilot has to deal with a lot more potential obstructions than an aircraft autopilot.

"We might as well just fly commercial" - and some "commercial" pilots think exactly the same way. My employer's chief pilot is type-qualified to fly most of the current Boeing and Airbus range, but what he really likes about flying is spending a Sunday tow-launching gliders at a local club, or doing an aerobatics display somewhere in the company's WWII Spitfire, not spending hours monitoring the glass cockpit display in an A330 at 35,000ft.

@tj1000: Sure, the automobile autopilot would be attractive to limited markets, but hardly to the automobile enthusiast who's the typical buyer of upscale cars.  (I won't even buy a car with automatic transmission.)  Nor do I think it would help much with drunk drivers, as the drunk would have to be sober enough to decide to turn on the autopilot.